Drop support for Node.js < 8

[rosen-vladimirov]

Is your feature request related to a problem? Please describe.
Node.js versions below 8 are deprecated for a long time. Currently, when Node.js 6 is used, CLI shows a warning that this version is deprecated. It is time to stop supporting these versions.

Describe the solution you'd like
In case Node.js version is below 8, stop the execution of all CLI commands.

Describe alternatives you've considered
None

Additional context
Supporting multiple versions of Node.js requires a lot of testing, so we should limit them.